This application is based on and incorporates herein by reference Japanese Patent Application No. 2001-182032 filed on Jun. 15, 2001.
1. Field of the Invention
The present invention relates to a capacitive moisture sensor. The sensor includes a pair of electrodes and a moisture-sensitive film, which is located between the electrodes. The dielectric constant of the film varies in response to ambient moisture. The ambient moisture is sensed on a basis of the capacitance change between the electrodes.
2. Description of the Related Art
A capacitive moisture sensor is utilized to measure indoor moisture for an air conditioner, outdoor moisture for meteorological observation, and so on. This type of capacitive moisture sensors are proposed in JP-B2-6-105235, JP-A-55-66749, and JP-A-60-166854. However, in the sensors of the publications, a bottom electrode, a moisture-sensitive film, and a thin top electrode, which is moisture permeable, are located in this order on a substrate, and the top electrode is exposed to the atmosphere. Therefore, the sensor is relatively poorly moisture-proof and has relatively poor durability. As a solution to the poor durability issue, a different type of capacitive moisture sensor is proposed. In the different type of sensor, an insulating film is located on a substrate, a pair of electrodes is located on the insulating film, and a moisture-sensitive film is located on the electrodes to cover the electrodes.
However, the different type of sensor has parasitic capacitance between each electrode and the substrate. In general, the capacitance change amount corresponding to ambient moisture change amount is relatively small in a capacitive moisture sensor, so the parasitic capacitance worsens the precision and accuracy in measurement of the ambient moisture.
The present invention has been made in view of the above aspects with an object to provide a capacitive moisture sensor that has a desired precision and accuracy in measurement of ambient moisture without exposing either electrode of a capacitor for the measurement to the atmosphere.
In the present invention, a capacitive moisture sensor includes a semiconductor substrate, which has a hole. A silicon oxide film is located to close the hole. A pair of electrodes is located on the silicon oxide film. Each electrode is in the shape of a comb, and the electrodes mesh with each other. A silicon nitride film is located on the electrodes to cover and protect the electrodes and on the silicon oxide film between the electrodes. A moisture-sensitive film, the dielectric constant of which varies in response to ambient moisture, is located on the silicon nitride film. The thickness of the substrate is substantially zero under the electrodes to eliminate the parasitic capacitance between each electrode and the substrate.